1. Field of the Invention
This invention relates generally to oscillators, and more particularly to an oscillator which is completely contained in a single integrated circuit chip.
2. Prior Art
A number of different types of oscillators are available for integrated circuit configurations. These integrated circuit oscillators exhibit extreme variations in frequency with variations in process parameters and with variations in the voltage level of their power supply. Those process parameters which provide the largest variations are the enhancement threshold voltage and the mobility and gate capacitance of field effect transistors (FET). Since the resistance of an FET does not vary significantly with variations in the voltage level of the power supply, the time required to charge or discharge each node of the oscillator circuit varies with variations in the voltage level of the power supply. Accordingly, it can be appreciated that variations in process parameters and variations in the voltage level of the power supply connected to the oscillator cause variations in the frequency of the output signal of the oscillator. Frequency variations of 20 to 1 can be expected from such prior known oscillators because of process parameter variations and power supply variations.
Previous oscillators also exhibited start up problems. That is, relatively small variations in the process parameters of prior known oscillators could result in that oscillator being incapable of starting. Also, race condition problems exist in some of these prior known oscillators. 132
Previous integrated circuit oscillators were also susceptible to the problem of having more than one stable oscillation frequency. Usually, this problem was encountered when the voltage level of the oscillator power supply dropped below a predetermined amplitude. This problem also resulted in some of the prior known integrated circuit oscillators when the power supply voltage was turned on and applied to the oscillator at a relatively slow rate.
Some of the prior known integrated circuit oscillators, particularly those in which the effects of some of the above mentioned problems were reduced, required a relatively large number of components. Some of these integrated circuit oscillators also required more than one timing capacitor, such that a change or adjustment of the nominal frequency of the oscillator required a change of more than one component or capacitor in that circuit.
Furthermore, some of these prior known integrated circuit oscillators required certain components which could not be integrated into a circuit chip. Usually, the timing resistance and capacitance devices in prior known oscillators are of a size which does not permit them to be in the form of an integrated circuit. It has been the practice in the past to determine the frequency of oscillation by the size of these timing components; that is, by the RC time constant of the circuit. If a relatively low frequency of oscillation is desired, the size of the timing components is relatively large, thereby rendering them unsuitable for inclusion in an integrated circuit.
It can be appreciated, therefore, that a need exists for an integrated circuit oscillator in which all of its components can be integrated into a circuit chip and which exhibits a relatively small variation in frequency with variations in process parameters and/or variations in power supply levels. A need also exists for such an oscillator which has no start up problems, only one stable oscillation frequency, and is not subject to race conditions.